6.06.2014

Dealing with Nested Documents in MongoDB and Talend (aka Baking a Cake)

Abstract:

MongoDB is a very popular document database that gives you a huge amount of flexibility when it comes to storing data. On the other hand, the traditional relational model is far less flexible in how data is stored - you're limited to columns and rows. Sometimes you want to go from a flexible model like MongoDB to a relational one, that's what this post attempts to explain using Talend/JaspersoftETL (ETL tools).

I do not want to get into the relational vs non-relational model argument in this post - it's only an example if you need to do this for some reason...I'm pro choice :)

Scenario:

We have a JSON document in MongoDB that looks like this:
 {  
      "id": "0001",  
      "type": "donut",  
      "name": "Cake",  
      "ppu": 0.55,  
      "batters":  
           {  
                "batter":  
                     [  
                          { "id": "1001", "type": "Regular" },  
                          { "id": "1002", "type": "Chocolate" },  
                          { "id": "1003", "type": "Blueberry" },  
                          { "id": "1004", "type": "Devil's Food" }  
                     ]  
           },  
      "topping":  
           [  
                { "id": "5001", "type": "None" },  
                { "id": "5002", "type": "Glazed" },  
                { "id": "5005", "type": "Sugar" },  
                { "id": "5007", "type": "Powdered Sugar" },  
                { "id": "5006", "type": "Chocolate with Sprinkles" },  
                { "id": "5003", "type": "Chocolate" },  
                { "id": "5004", "type": "Maple" }  
           ]  
 }  
We want to put the document into three different tables,  for use in a relational database:

Main Table:

ID TYPE NAME PPU
0001 donut cake 0.55

Batter Table:

BATTERID BATTERTYPE CAKEID
1001 regular 0001
1002 chocolate 0001
1003 blueberry 0001
1004 devil's food 0001

Topping Table:

TOPPINGID TOPPINGTYPE CAKEID
5001 None 0001
5002 Glazed 0001
5005 Sugar 0001
5007 Powdered Sugar 0001
5006 Chocolate with Sprinkles 0001
5003 Chocolate 0001
5004 Maple 0001

Talend tMongoDBInput Component

The tMongoDBInput component is what we'll use to extract the data from MongoDB and input into Talend. This is fairly straightforward, you can configure the connection and then you will have to map some fields. We map id, type, name, ppu as regular fields. The batter and topping fields are somewhat special. 

Toppings:
The toppings field will be mapped a string and will contain an object in each row that we'll need to deal with later

Batters:
Batters is a nested document within another nested document and we'll need to define a parent node path. tMongoDBInput does not allow you to use node notation to select fields (like batters.batter) so you'll define a parent node like this:
In the end the schema for tMongoDBInput will look like this:

tMap Component - Splitting

Now we'll use a tMap component to split the outputs into three (main, topping, batter)



tExtractJSONFields - Expanding Documents
So now we have to deal with the special fields, batter and topping - each one of them contains a list that we want to denormalize into it's own rows. We can use the tExtractJSONFields component for this. The component has a couple of key things: schema, Loop XPath Query and XPath query. This determines what fields to map.

Schema:

Then set the Xpath queries:

And finally glue it all together:
Note that we have 3 input rows, then for each batter and topping we are extracting.

If you are interested in this example, you can download it here:

  • Data: This is a JSON object you can import with mongoimport (like  mongoimport --jsonArray --collection nestedSample < doc.json)
  • JaspersoftETL Job: This job works with JaspersoftETL 5.4.1 Big Data Edition (or presumably the Open Source Big Data edition of Talend)

11 comments:

Stas P. said...

Very timely post, Ernesto! I was just looking for a way to do this

Ernesto Ongaro said...

Usually you shouldn't believe in coincidences - someone asked me a question about this today so assuming it's all related :)

vishal wagh said...

This is awesome! also, is there a way to read and create a relational json doc using tInputJosn and tWriteJsonFields respectively. Currently it just supports one loop element :(

Ernesto Ongaro said...

Hi Vishal, the tExtractJSONFields component is not specific to MongoDB, would that work for your case?

vishal wagh said...

Thanks Ernesto! tExtractJSONFields works great. The main problem we are facing is creating a relational JSON output. The JSON Output components just supports only one loop element in the schema, is there a way to generate relational JSON doc in Talend?

On Tap said...

Vish or Ernesto,

Did anyone ever figure out how to deal with the "only one loop element in a schema" issue? I have been pulling my hair out trying to solve this issue. Any help would be appreciated.

Matt

Anonymous said...

Timely post!! Highly appreciated!!

jake george said...


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RAJKUMAR said...

Hi i have json like this.

"marks":{
"sem1" :{
"mark1":10,
"total":100
},
"sem2":{
"mark2":20,
"total":200
},
"sem3":{
"mark2":30,
"total":300
}
}

I need result like

mark total sem

10 100 sem1
20 200 sem2
30 300 sem3

how can i achive above format using monogodb query.query is jaspersoft related means very useful

Stephen MacKenzie said...

Hi,

Very nice article. I would love an article on how to go the other way. i.e. Going from multiple related relational tables into a complex JSON document for mongoDB.

Regards

Stephen

Sriram Venkatesan said...

Hi - Thanks for nice article

How do you get CAKE_ID in the batter table? I am trying a similar work to move data from mongodb to mysql and need the ID for foreign key reference

Is there any special setting needed in tMap or any other component?

Thanks

Sriram AV